The present invention concerns a method and system for determining anemobaroclinometric parameters on board an aircraft.
Present-day anemometry on board aircraft is based on the measurement of parameters relating to air, such as static pressure, total pressure and impact temperature, assisted by probes and sensors.
The probes are protuberant devices on the skin of the aircraft, which therefore have a certain number of drawbacks.
From the point of view of aerodynamics, the probes disturb the flow of air and can interfere with the engine air intakes. They also increase drag.
From the electromagnetic point of view, they generate a radar signature which undermines the stealth of the aircraft, and require de-icing, which undermines infrared concealment and consumes energy.
The measurements are, moreover, carried out locally in aerodynamically disturbed areas and therefore necessitate significant corrections, which are modelled during flight trials but which do not afford all the precision desirable in all fields of flight (for example in post-stalling).
Notwithstanding a high precision of measurement in the sensors, the performance of the anemobaroclinometric function is impaired by non-reproducibility phenomena and wearing of the probes, and is limited by residual modelling errors.
Increased operational constraints of stealth and maneuverability are pushing current anemometry to its limits. If the protuberance of the probes is to be eliminated in order to satisfy the constraints of stealth, the modelling complexity then becomes all the more tricky if the aircraft is also to be maneuverability.
The present invention aims to overcome these drawbacks.
To this end, the object of the invention is, first of all, a method of determining anemobaroclinometric parameters on board an aircraft, characterized by the fact that measurements are made of:
the air speed vector (Vp); PA1 the static pressure (Ps); PA1 the impact temperature (Ti);
and that the other parameters are calculated on the basis of the aforementioned measurements.
It will, indeed, be seen hereinafter that all the parameters can be obtained on the basis of the three aforementioned measured parameters.
The choice of these three parameters has the advantage that they can he obtained by non-protuberant probes.
In particular, the air speed vector can be measured by means of longitudinal laser anemometry.
Doppler longitudinal anemometry proceeds by measuring the Doppler shift f to which a monochromatic light wave of wavelength .lambda. is subjected when it is backscattered by aerosols in suspension in the atmosphere. This shift equals .DELTA.f=2v/.lambda., where V designates the component of the air speed vector along the sight line.
The measurement of the Doppler shift therefore gives the component of the air speed vector through a rigorously linear law in the speed range in question, and whose scaling factor depends only on the wavelength of the laser, which is generally known with high precision and does not change over time. At a wavelength of 2 .mu.m, the scaling factor is approximately 1 MHz per m/s.
The measurement is carried out at several tens of meters (50 to 100 meters) upstream of the aircraft in an undisturbed area within a measurement volume whose typical length is around ten meters.
The three components of the true air speed vector can be obtained by means of several sight lines. The measurement areas on the various axes are then discontinuous but their distance is less than the scale of atmospheric turbulence.
Although Doppler longitudinal anemometry has the advantage of measuring the air speed in a non-disturbed aerodynamic area, it is possible to use other optical methods which do not use a protuberant probe.
Thus a fringing laser anemometer, or a so-called "twin-layer" or "flight-time" anemometer could be used.
It is theoretically possible to measure the speed vector projection moduli of only over three measurement axes, but a redundant measurement over four axes is preferably used.
If a measurement of the conventional type is also made of the ground speed vector, the wind vector can be derived therefrom, since the air speed vector is known.
Knowledge of the wind speed vector is thus of particular interest: it is in fact possible, between two successive determinations of the true air speed vector, to determine an air speed vector calculated on the basis of the ground speed vector and the last wind vector obtained.
Another object of the present invention is an anemobaroclinometric system for implementing the method described above, characterized by the fact that it comprises at least one means of measuring the air speed vector (Vp), at least one means of measuring static pressure (Ps), at least one means of measuring the impact temperature (Ti), and at least one computer set up to calculate the other anemobaroclinometric parameters on the basis of the air speed, static pressure and impact temperature.
In a preferred embodiment, the means of measuring the air speed comprise two longitudinal laser anemometers with four axes of measurement.
Advantageously, three computers are provided, each receiving the eight speed projection measurements, at least one static pressure measurement and at least one impact temperature measurement.